Over the years, a substantial number of flow sensors have been developed that rely on a flow-activated element and one or more electrodes responsive to the flow-activated element. Some of those flow sensors rely on a range of conductivity of the fluid, thereby rendering them useless for sensing the flow of air and other gases and for sensing highly resistive or insulating liquids such as petroleum and its derivative liquids.
Some flow sensors involving sensing electrodes rely for their operation on the changes of capacitance occurring at a single sending electrode or electrode pair caused by a flow-activated element. Some of such flow sensors may be affected only incidentally by conductivity and other properties of the liquids to be monitored, but the intended dominant effect in such sensors is the capacitance changes caused by the flow-activated element at the sensing electrode(s).
"Capacitive flow sensors" (as they are called here) undergo changing capacitance at one or more sensing electrodes in response to flow-activated members in the sensing region. Among various known capacitive flow sensors are those which sense flow-induced capacitance changes at the sensing electrode(s) caused by an orbiting ball, and those which sense successive slender vanes of a flow-activated rotor such as a turbine rotor or a paddle wheel. A capacitive flow sensor may involve an array of sensing electrodes, or a pair of sensing electrodes, or a single localized sensing electrode, which are referred to below as "sensing electrode means".
An excellent form of capacitive flow sensor is one that relies on an inductance connected in series with the capacitance of capacitive sensing electrode means, disclosed in U.S. patent application Ser. No. 07/632,520,filed Feb. 13, 1991 and its continuation Ser. No. 08/020,908, filed Feb. 22, 1993 by Murray F. Feller. The sensing capacitance and the series-connected inductance form a series-resonant circuit, and the changing impedance of the series-resonant circuit due to alternating proximity and distance of each sensed flow-activated element relative to the sensing electrode modulates the amplitude of an excitation signal. Notably, that flow sensor is highly sensitive to the sweep of the edges of slender vanes or blades of a flow-driven rotor past a very small sensing electrode. A high degree of sensitivity is valuable in detection of the characteristically small capacitance changes at such small parts.
Maintaining that flow sensor in stable operation may require adjustments to take into account varied operating conditions, and stable operation may warrant limiting the Q of the series-resonant circuit which consequently limits the sensitivity of the apparatus. Moreover, for achieving greatest sensitivity there, a single sensing electrode is placed as close as is feasible to the path of flow-activated elements, consistent with practical limitations. The inherent shunt or stray capacitance of the sensing electrode (capacitance that is not caused to vary by the flow-activated elements) is minimized. This concern imposes its own exacting constraints on the practical design and construction of the sensing electrode and its connection to the series inductance.